1. Field of the Invention
The present invention relates to a membrane separation device and a method for operating the same, where the membrane separation device includes a separation membrane immersed in water to be treated, and an air diffusion device positioned beneath the separation membrane, and provides treated water that has permeated through the separation membrane while diffusing air from the air diffusion device towards the separation membrane.
2. Description of the Related Art
Conventionally, a membrane bioreactor process has been widely used as a method for treating organic wastewater and the like (hereinafter called “water to be treated”), in which organic substances are biodegraded by a purification treatment using microorganisms in activated sludge, and separated water is obtained by solid-liquid separation using a separation membrane. Various methods using a membrane separation device having a separation membrane such as microfiltration membrane, ultrafiltration membrane, or the like have been evaluated as such solid-liquid separation of water to be treated.
Such a membrane separation device is disposed and immersed in the water to be treated, and provided with an air diffusion device positioned beneath the separation membrane thereof in order to prevent filtration efficiency from decreasing by contents adhere to the separation membrane surface, such as the activated sludge itself in a treatment tank, impurities or the like in the water to be treated flowing into the treatment tank, as well as so called fouling substances such as polymer solute from the microorganisms or the water to be treated, colloids, and fine solid matters, or the like. The air diffusion device diffuses air or the like into the tank so as to cause a vibration effect of the separation membrane and a stirring effect of an upward current of air bubbles and the water to be treated, thereby suppressing deposits of solid contents and the like on the separation membrane surface and removing attached solid contents or the like from the separation membrane surface.
Conventionally, the amount of the diffused air supplied from the air diffusion device is set to exceed the minimum amount required for washing the membrane surface such that the separation membrane surface is not easily clogged even under a condition in which the separation membrane surface can otherwise be easily clogged by the influence of sludge properties or the like, which disadvantageously increases the cost of electrical power for air diffusing.
Considering the fact that approximately half of the operating cost required for treatment facilities employing the membrane bioreactor process is being spent on the air diffusion, it is an important issue to reduce the total amount of air diffusion while preventing the clogging of the separation membrane.
In order to reduce the energy cost for the amount of aeration air, Japanese Unexamined Patent Publication No. 2005-144291 proposes a method for controlling an amount of aeration air, which (i) monitors the transmembrane pressure difference of the membrane separation device, (ii) sets the amount of aeration air when the transmembrane pressure difference is within a normal level range excluding an unsteady state of the transmembrane pressure difference to a predetermined permissible steady-state value, and (iii) controls the amount of aeration air supplied from the air diffusion device to increase in the unsteady state when the transmembrane pressure difference suddenly rises above the predetermined value.
This Patent Publication also describes that the amount of aeration air when the behavior of the transmembrane pressure difference is in a steady-state level is set to an allowable lower limit which is such a lower limit of the amount of aeration air, determined by a preliminary test in advance, that the transmembrane pressure difference drastically increases if the amount of aeration air is reduced below this lower limit, and that whether or not the transmembrane pressure difference increases above the predetermined value is evaluated by a rate of increase in the transmembrane pressure difference (for example, expressed as kPa/day).
However, since the transmembrane pressure difference of the separation membrane tends to increase over time, and properties of the water to be treated flowing into the sludge treatment device change according to the season and time, clogging of the separation membrane also varies accordingly. Thus, operating at the lower limit for an extended period of time is realistically challenging, and rather it may cause premature clogging of the separation membrane.
Furthermore, the evaluation using the rate of increase in the transmembrane pressure difference expressed by kPa/day takes at least one day to determine if the transmembrane pressure difference has increased above the predetermined value, and thus the clogging may worsen during that time period.
Although various experiments other than the method disclosed in Japanese Unexamined Patent Publication No. 2005-144291 have been conducted, it is still very challenging to effectively reduce the electric power cost.